In various types of image forming apparatuses such as laser printers, copiers, and facsimile machines, an image forming operation thereof is carried out via processes each referred to as charging, exposure, development, transfer, and fixing.
Specifically, initially, by irradiation of light beams (laser beams) modulated based on image data, on the surface of a charged photoreceptor drum, an electrostatic latent image of an image to be subjected to image formation is formed, and then this electrostatic latent image is developed as a toner image using a developing device. The thus-formed toner image is transferred onto a transfer medium (a recording paper) by a transfer roller. Then, in a fixing device, the toner image on the recording paper is fixed by fixing heat to form an image on the recording paper.
When a toner image is fixed onto a recording paper, there occurs a phenomenon such that moisture in the recording paper is removed by fixing heat and thereby the recording medium is contracted. Especially when double-sided image formation is carried out, there is produced a problem such that due to recording paper contraction during fixing of a first face (front face), the image sizes of a first face (front face) and a second face (reverse face) of a recording paper become different and thereby the image sizes of the front face and the reverse face are misaligned.
In such a case, when pixel clock cycle is changed and also the rotation speed of a polygon mirror is changed, the front and reverse image sizes can be matched, for example, via fine adjustment of the image magnification of the reverse face side. This image magnification adjustment by the polygon mirror rotational speed change produces an advantage in which no image quality degradation occurs, but there is noted the problem that it takes a certain period of time to change the rotation speed of the polygon mirror.
Further, in the same manner, image processing also makes it possible that via fine adjustment of the image magnification of the reverse face side, the front and reverse image sizes are matched. This image magnification adjustment via image processing can be processed for a short time differently from the case of a polygon mirror, but in contract, there is a problem in which thinning-out and interpolation cause image quality degradation.
Incidentally, other than thermal contraction of a recording paper as described above, the operator occasionally gives instructions of slight enlargement or reduction of an image. Such a case can also be handled by the polygon mirror rotational speed change or image processing.
There exist, for example, Patent Documents 1 and 2 as described below referring to the control of the rotation speed of a polygon mirror in this manner.
Incidentally, a polygon mirror is driven by PLL (Phase Locked Loop) control, being rotated at a high speed of several tens of thousands of rotations per minute. Further, when the rotation speed is changed depending on the fine adjustment of image magnification as described above, it is necessary to take a certain period of time (stabilizing time) until both the PLL control and the rotation of the polygon mirror reach a stable state.
On the other hand, in a recent image forming apparatus, high-speed processing is demanded. With a decrease in paper feeding interval within an image forming apparatus, paper interval time, in which switching from image formation on the recording paper front face to image formation on the recording paper reverse face is carried out, is also decreased. Herein, the sheer interval time refers to a time in which no recording paper exists in the exposure section or the transfer section, namely, until the front edge of a next recording paper arrives after the tailed edge of a preceding recording paper conveyed during image formation has passed.
Therefore, in the case where image magnification adjustment is carried out by polygon mirror rotational speed change, when the stabilizing time due to the polygon mirror rotational speed change is more than the above paper interval time, it is necessary to decrease the number of circulating paper (the number of recording paper circulating in an image forming apparatus at a time) during double-sided image formation to extend the paper interval time.
In this case, for example, when such a paper interval time is extended via a decrease from 4 sheets of paper to 3 sheets of paper in the number of circulating paper, the productivity of the image forming apparatus is decreased to ¾.
In the techniques described in Patent Documents 1 and 2, the above stabilizing time has been attempted to decrease by optimizing the control of a polygon mirror PLL-driven via elimination of unnecessary processings.
However, with a demand for the productivity of an image forming apparatus, the paper interval time has been decreased more and more and also the reduction of the stabilizing time of a polygon mirror, as described above, is limited, whereby the stabilizing time of the polygon mirror occasionally exceeds the paper interval time depending on productivity, and thereby, the stabilizing time of the polygon mirror has inhibited productivity enhancement.
On the other hand, without the change of the rotation speed of a polygon mirror, image magnification adjustment can be carried out via image processing of image data. However, this method has occasionally produced the problem of image quality degradation.
Namely, in the image magnification adjustment, no method to optimize productivity enhancement (productivity decrease prevention) and image quality maintenance (image quality degradation prevention) has been established.
An object of the present invention is to provide an image forming apparatus in which in image magnification adjustment in an apparatus to form an image by polygon mirror scanning of light beams, image formation can be carried out with excellent image quality in the range of no decrease in productivity.